1. Field of the Invention
The present invention relates to the field of semiconductor inspection. More particularly, the present invention provides an automated method and apparatus for focusing a microscope on semiconductor wafers and other specimens.
2. Description of Related Art
In the process of manufacturing semiconductors, a series of photolithographic steps are interspersed with chemical or other treatments to define circuits in and on a semiconductor wafer. For example, a lithographic step may be used to form a pattern of photoresist on the wafer to expose (or protect) the portions of the wafer that are to be subjected to a subsequent treatment step. Each of the processing steps executed in the fabrication of the semiconductor devices on a wafer must be executed with extreme precision or the resulting devices will not be functional. For example variations in thicknesses or line widths of only a micrometer (micron) can destroy an entire wafer.
Microscopes have long been used in the inspection and fabrication of semiconductor devices and other devices which require precision in their manufacturing processes. Such microscopes may be used for examining the semiconductor device for particulates, physical defects, or other inspection tasks during the production process.
It is desirable to provide automatic focusing systems for the microscopes used in these processes and, accordingly, a wide variety of automatic focusing systems have been proposed. For example, U.S. Pat. No. 4,600,832 (Grund) describes an autofocusing system in which sharply defined edges on the specimen are used to focus the microscope. An "edge value" is determined, and the edge value is minimized, at which time the microscope is assumed to be in focus. The method assumes, of course, that the specimen contains sharply defined edges in the first event. Further, it assumes that diffracted edge profiles from the lines do not interfere with the focus calculation.
U.S. Pat. No. 4,639,587 (Chadwick et al.) discloses an automatic focusing system in which a chopper alternately admits light to one of two optical paths. Light from the two optical paths is directed at the target article to cast alternating images thereon. The alternating images are ultimately "subtracted" and the device focus is adjusted until the difference is essentially zero, at which time the image is assumed to be in focus.
The device described by Chadwick et al. has a number of disadvantages. The system described by Chadwick is believed by the inventors herein to be unduly sensitive to opto-mechanical alignment, long term stability, optical path through the "flipping pupil", and the like, due to the complexity of the system. Further, the system is believed to be difficult to focus on dark samples, i.e., samples which absorb significant amounts of light at a wavelength of interest.
U.S. Pat. No. 4,595,829 (Neumann et al.) discloses an autofocusing system in which a laser beam is directed at the surface of the sample through the objective lens. Markers 10a and 10b (see FIGS. 2 and 3 of Neumann et al.) are used to locate the position of an "invisible" measuring spot 16. The system described in a Neumann et al. is believed to be difficult to utilize when a sample absorbs light at the wavelength of a measuring beam used for focusing. Further, focusing is based on a beam projected through an "edge" of a lens, resulting in inaccuracies in the focus. Further, the system must be re-adjusted for each objective.
Another automatic focusing system is described in U.S. Pat. Nos. 4,556,317, 4,618,938 and 4,644,172 (Sandland et al., see especially column 19, line 49 et. seq. of 4,644,172). The automatic focusing system of Sandland et al. directs 600 nanometer wavelength light at a pupil stop which contains an eccentric pin hole aperture which offsets an image location. This system suffers from many of the disadvantages of Chadwick U.S. Pat. No. 4,639,587. The offset is adjusted by adjusting the focus of the microscope until the image of the projection reticle falls on the reticle so that the image of the reticle pattern is coincident with the reticle pattern itself.
Identical disclosure is found in U.S. Pat. No. 4,604,910 (Chadwick et al.). See also U.S. Pat. No. 4,347,001 which discloses another commonly used technique for focusing an inspection microscope at Column 10, line 39 et. seq.
Other related patents in the field include U.S. Pat. Nos. 4,656,358 (Divens et al.), 4,433,235 (Akiyama et al.), 4,448,532 (Joseph et al.), 4,532,650 (Wihl et al.), 4,555,798 (Broadbent, Jr. et al.), 4,633,504 (Wihl), 4,363,962 (Shida), 4,447,717 (Nohda), 4,363,961 (Okada et al.), 4,609,814 (Nobuaki et al.), 4,701,606 (Tanimoto et al.), 4,705,940 (Kohno), 4,687,913 (Chaban) and 4,663,732 (Robinson).